Knowledge of the mechanisms by which environmental factors influence seasonal reproductive patterns in mammals is incomplete, but current evidence suggests that pituitary gonadotropin secretion may be altered. A regulatory role of the Pineal gland in the alteration of hypothalamic and pituitary function has been suggested. The main objective of our plan is to develop a working model in pony mares, by which the seasonal influence on reproductive function can be studied. The long-term goal toward which this proposal is directed is a better understanding of the events which regulate hypothalamic and pituitary function. The knowledge is essential for a complete understanding of central nervous system-gonadal feedback mechanisms. The main hypothesis to be tested is that the hypothalamo-pituitary axis plays a pivotal role in response to pineal-mediated seasonal factors and/or ovarian feedback. Central to this hypothesis is the concept that, in mares, the ovaries are largely non-functional throughout anestrus, but play a major feedback role during the characteristic period of sexual recrudesence. To this end, pineal, hypothalamic, pituitary and ovarian hormones will be quantified in the peripheral circulation and cerebrospinal fluid during varying environmental conditions and in ovariectomized and/or pinealectomized animals.